Paper webs or sheets, sometimes called tissue or paper tissue webs or sheets, find extensive use in modern society. Such items as facial and toilet tissues are staple items of commerce. It has long been recognized that four important physical attributes of these products are their strength, their softness, their absorbency, including their absorbency for aqueous systems; and their lint resistance, including their lint resistance when wet. Research and development efforts have been directed to the improvement of each of these attributes without seriously affecting the others as well as to the improvement of two or three attributes simultaneously.
Strength is the ability of the product, and its constituent webs, to maintain physical integrity and to resist tearing, bursting, and shredding under use conditions, particularly when wet. Softness is the tactile sensation perceived by the consumer as he/she holds a particular product, rubs it across his/her skin, or crumples it within his/her hand. This tactile sensation is provided by a combination of several physical properties. Important physical properties related to softness are generally considered by those skilled in the art to be the stiffness, the surface smoothness and lubricity of the paper web from which the product is made. Stiffness, in turn, is usually considered to be directly dependent on the dry tensile strength of the web and the stiffness of the fibers which make up the web.
Absorbency is the measure of the ability of a product, and its constituent webs, to absorb quantities of liquid, particularly aqueous solutions or dispersions. Overall absorbency as perceived by the consumer is generally considered to be a combination of the total quantity of liquid a given mass of multi-ply facial tissue paper will absorb at saturation as well as the rate at which the mass absorbs the liquid.
Lint resistance is the ability of the fibrous product, and its constituent webs, to bind together under use conditions, including when wet. In other words, the higher the lint resistance is, the lower the propensity of the web to lint will be.
The use of wet strength resins to enhance the strength of a paper web is widely known. For example, Westfelt described a number of such materials and discussed their chemistry in Cellulose Chemistry and Technology, Volume 13, at pages 813-825 (1979). Freimark et al. in U.S. Pat. No. 3,755,220 issued Aug. 28, 1973 mention that certain chemical additives known as debonding agents interfere with the natural fiber-to-fiber bonding that occurs during sheet formation in paper making processes. This reduction in bonding leads to a softer, or less harsh, sheet of paper. Freimark et al. go on to teach the use of wet strength resins in conjunction with the use of debonding agents to off-set the undesirable effects of the debonding agents. These debonding agents do reduce both dry tensile strength and wet tensile strength.
Shaw, in U.S. Pat. No. 3,821,068, issued Jun. 28, 1974, also teaches that chemical debonders can be used to reduce the stiffness, and thus enhance the softness, of a tissue paper web.
Chemical debonding agents have been disclosed in various references such as U.S. Pat. No. 3,554,862, issued to Hervey et al. on Jan. 12, 1971. These materials include quaternary ammonium salts such as cocotrimethylammonium chloride, oleyltrimethylammonium chloride, di(hydrogenated)tallow dimethyl ammonium chloride and stearyltrimethyl ammonium chloride.
Emanuelsson et al., in U.S. Pat. No. 4,144,122, issued Mar. 13, 1979, teach the use of complex quaternary ammonium compounds such as bis(alkoxy(2-hydroxy)propylene) quaternary ammonium chlorides to soften webs. These authors also attempt to overcome any decrease in absorbency caused by the debonders through the use of nonionic surfactants such as ethylene oxide and propylene oxide adducts of fatty alcohols.
Armak Company, of Chicago, Ill., in their bulletin 76-17 (1977) disclose the use of dimethyl di(hydrogenated)tallow ammonium chloride in combination with fatty acid esters of polyoxyethylene glycols to impart both softness and absorbency to tissue paper webs.
One exemplary result of research directed toward improved paper webs is described in U.S. Pat. No. 3,301,746, issued to Sanford and Sisson on Jan. 31, 1967. Despite the high quality of paper webs made by the process described in this patent, and despite the commercial success of products formed from these webs, research efforts directed to finding improved products have continued.
For example, Becker et al. in U.S. Pat. No. 4,158,594, issued Jan. 19, 1979, describe a method they contend will form a strong, soft, fibrous sheet. More specifically, they teach that the strength of a tissue paper web (which may have been softened by the addition of chemical debonding agents) can be enhanced by adhering, during processing, one surface of the web to a creping surface in a fine patterned arrangement by a bonding material (such as an acrylic latex rubber emulsion, a water soluble resin, or an elastomeric bonding material) which has been adhered to one surface of the web and to the creping surface in the fine patterned arrangement, and creping the web from the creping surface to form a sheet material.
Conventional quaternary ammonium compounds such as the well known dialkyl dimethyl ammonium salts (e.g. ditallow dimethyl ammonium chloride, ditallow dimethyl ammonium methyl sulfate, di(hydrogenated)tallow dimethyl ammonium chloride etc . . . ) are effective chemical debonding agents. However, these quaternary ammonium compounds are hydrophobic, and can adversely affect the absorbency of the treated paper webs. Applicants have discovered that mixing the quaternary ammonium compound with a polyhydroxy compound (e.g., glycerol, polyglycerols or polyethylene glycols) will enhance both softness and absorbency rate of fibrous cellulose materials.
Unfortunately the use of chemical softening compositions comprising a quaternary ammonium compound and a polyhydroxy compound can decrease the strength and the lint resistance of the treated paper webs. Applicants have discovered that both strength and lint resistance can be improved through the use of suitable binder materials such as wet and dry strength resins and retention aid resins known in the paper making art.
The present invention is applicable to tissue paper in general, but particularily applicable to multi-ply, multi-layered tissue paper products such as those described in U.S. Pat. No. 3,994,771, issued to Morgan Jr. et al. on Nov. 30, 1976, and in U.S. Pat. No. 4,300,981, Carstens, issued Nov. 17, 1981, both of which are incorporated herein by reference.
The multi-ply facial tissue paper products of the present invention contain an effective amount of wet strength binders, permanent and/or temporary, combined with dry strength binders to control linting and/or to offset the loss in tensile strength, if any, resulting from the use of the chemical softening compositions. Unexpectedly, it has been found that the combination of both wet strength binders, permanent and/or temporary, and dry strength binders improves the retention of the chemical softening composition in the sheet. This results in improved softness of the multi-ply facial tissue paper product. This softness improvement can be further understood by noting improvement in one or more of the following paper properties: the flexibility, the slip-stick coefficient of friction and/or physiological surface smoothness (see Ampulski et al., 1991 International Paper Physics Conference Proceedings, book 1, page 19-30, incorporated herein by reference). The increased softener retention is accompanied by little or no additional tensile loss versus a tissue paper sheet formed without the combination of binder materials. This maximizes the softening capabilities with minimal additional negative impacts on the product and process.
It is an object of this invention to provide soft, absorbent and lint resistant multi-ply facial tissue paper products.
It is also a further object of this invention to provide a process for making soft, absorbent, lint resistant multi-ply facial tissue paper products.
These and other objects are obtained using the present invention, as will become readily apparent from a reading of the following disclosure.